A Preliminary Trophic Model of the Continental Shelf, South-western Gulf of Mexico

A preliminary mass-balance trophic model was constructed to determine the flow of energy in a community of fish and invertebrates on the continental shelf of the south-western Gulf of Mexico. Input parameters were taken from the literature, except for the biomass of fish groups which was obtained from trawl surveys in the study area. The model consists of 12 fish groups, five invertebrate groups, phytoplankton and detritus. Results indicate an imbalance between primary production and consumption, with only about 10% of primary production being consumed in the water column. Most of the primary production is exported to detritus which forms the basis of the food-web, with a detritivory/herbivory ratio of 2•5. Benthic invertebrates play a significant role in transferring energy from detritus to higher trophic levels. Eight discrete trophic levels were found, with very reduced flow at levels higher than the fifth. The highest fractional trophic level is 4•3, and consists of sharks. Analysis of mixed trophic impacts showed that detritus and lower trophic levels have a significant positive impact on other groups in the system. Mean transfer efficiency is 9•2%. Some whole system properties are also given. Of net primary production, 6•7% is required to sustain current catch levels, suggesting that the resources in this area are fully exploited.